This invention relates to compressible printing blankets and method of manufacturing such blankets. The type of blanket referred to herein is laminated and is used primarily in offset lithographic printing, but may also find utility in other fields of printing.
The use of blankets in offset lithography has the primary function of transferring the ink from the printing plate to the paper. Printing blankets are very carefully designed so that the surface of the blanket is not damaged either by the mechanical contact with the parts of the press or by chemical reaction with the ink ingredients. The repeated contacts cause a certain amount of compression of the blanket which must be within proper limits so that the image is properly reproduced without causing permanent deformation of the blanket. The blanket must be capable of eventually returning to its original thickness and provide constant transfer of the image throughout its life.
In addition to the inherent variations of the printing apparatus, conventional blankets may also have minute variations in thickness which are formed during the manufacturing steps. The existing high or low spots in such a blanket may, therefore, create uneven reproductions on the finished product.
Printing blankets utilize a base material to give the blanket integrity, and woven fabrics are preferred for this purpose. The base may consist of one, two, three, or more layers of fabric. The working surface, by which is meant the surface that actually contacts the ink, is usually an elastomeric layer made of natural or synthetic rubber which is applied over the base layer. This is usually done by calendering or spreading rubber in successively thin layers until a desired thickness of rubber has been deposited, after which the assembly is cured to provide the finished blanket. Such a blanket is perfectly acceptable for many kinds of printing, but often lacks the necessary compressibility for other operations. It is desirable, however, to have more highly compressible blankets available.
It is difficult to obtain this improved compressibility by the standard construction described above because the rubber material, while it is highly elastomeric, cannot be compressed in a direction at right angles to its surface without causing a distortion or stretch of the blanket. If irregularities exist in the printing plate, the presses, or the paper, then the compression to which the blanket is exposed will vary during the operation. Other approaches to compressible blanket design must be made.
Blanket manufacturers have introduced products in the past in an effort to create additional compressibility, such as by the use of sponge rubber, textile fibers, special fabric layers, felt, or microspheres. Some of these are described in U.S. Pat. Nos. 2,792,322; 3,147,698; 3,285,799; 3,652,376, and 3,700,541.